Proposed work includes the following: 1. Examination of the capacity of JC virus to transform hamster and human cells and comparison of JC virus in this respect with SV40. 2. Comparison of JC virus, BK virus and SV40 nucleic acids using nucleic acid hybridization techniques. 3. Continued efforts to cultivate JC virus in cells other than human fetal glial cells. 4. Continued studies aimed at recognizing primary infections caused by JC virus.